This invention generally relates to spray dryers and more particularly to methods and equipment for drying particles produced by spray drying.
Spray drying is commonly used in the production of particles for many applications, including food, cosmetics, fertilizers, dyes, and abrasives. Spray drying can be tailored to create a wide spectrum of particle sizes, including microparticles. Spray dried particles are useful in a variety of biomedical and pharmaceutical applications, such as the delivery of therapeutic and diagnostic agents, as described for example in U.S. Pat. No. 5,853,698 to Straub et al., U.S. Pat. No. 5,855,913 to Hanes et al., and U.S. Pat. No. 5,622,657 to Takada et al.
In a typical process for making particles using a spray drying process, a solid forming material, such as a polymer, which is intended to form the bulk of the particle, is dissolved in an appropriate solvent to form a solution. Alternatively, the material can be suspended or emulsified in a non-solvent to form a suspension or emulsion. Other components, such as drugs, imaging agents, or pore forming agents, optionally are added at this stage. The solution then is atomized to form a fine mist of droplets. The droplets immediately enter a drying chamber where they contact a drying gas. The solvent is evaporated from the droplets into the drying gas to solidify the droplets, thereby forming particles. The particles then are separated from the drying gas and collected.
In scaling up such a spray drying process, for example from the laboratory or pilot plant scale to the commercial plant scale, certain disadvantages may be encountered. For example, if the drying efficiency is not adequately scaled, the solvent content of the product particles may increase undesirably. While increasing the drying capacity or drying rate should compensate for this insufficient drying, the increased drying rate may induce other problems. For example, it has been observed that increasing the drying rate results in unsuitable particle morphology and/or size distribution for some product particles, such as those having critically defined performance specifications. The change in drying rate may, for instance, alter the way in which the solid-forming material precipitates as the solvent is evaporated, thereby changing the structure (e.g., porosity) of the particle to be out of specification, rendering the particle unable to properly contain and deliver a diagnostic or therapeutic agent. Furthermore, changing the drying rate by reducing the flowrate (and consequently the velocity) of the drying gas may substantially reduce the product yield.
Even in cases where particle morphology and size distribution are less critical, scaling up the drying efficiency may require undesirably large increases in the size of process equipment, such as the drying chamber, drying gas source, and drying gas heater. The drying capacity generally is a function of the drying gas temperature, flowrate, pressure and solvent composition. Moreover, larger capacity equipment generally requires more plant space. It is desirable to minimize the capital investment and space required to scale up a production process.
It is therefore an object of the present invention to provide a method and apparatus for effectively drying particles made by spray drying.
It is another object of the present invention to provide a method and apparatus for spray drying that incorporates a drying process providing improved drying of the particles without detrimentally affecting product yield.
It is a further object of the present invention to provide an apparatus for drying spray dried particles that is relatively compact and inexpensive.